A conventional rechargeable battery constructed such that the required electrical capacity is obtained by connecting and integrally linking a plurality of cells into a battery pack is structured as shown in FIG. 16, for example. This rechargeable battery is an example of a battery pack made up of sealed alkaline rechargeable batteries, and the cells 1a to 1j that make up this battery pack are constructed as shown in FIG. 15.
In FIG. 15, the cell 1 is constructed such that a group of electrode plates 7, comprising positive and negative electrode plates layered with separators between them, is housed in a battery housing 2 along with electrolyte, the opening in the battery housing 2 is closed off with a cover 6 provided with a safety vent 5, and a positive electrode terminal 3, which is connected to leads taken from the positive electrode plates of the group of electrode plates 7, and a negative electrode terminal 4, which is connected to leads 9 taken from the negative electrode plates, are attached to the cover 6.
When the battery pack is put together, as shown in FIG. 16, the plurality of cells 1a to 1j are abutted against each other between the long (wide) sides of the battery housings 2, and the end plates 32, which are abutted against the outsides of the battery housings located at the ends, are bound together with binding bands 33 to integrally link the cells 1a to 1j. The positive electrode terminals 3 and negative electrode terminals 4 between linked adjacent cells 1 are connected by connector plates 31, and the cells 1 are connected in series. When the battery housings are linked, ribs 8 formed vertically on the long sides of the battery housings 2 are aligned with the ribs 8 on the adjacent battery housings 2, forming coolant passages that open above and below the battery housings 2 between the paired ribs 8.
A rechargeable battery generates joule heat and reaction heat through the chemical reaction that accompanies charging and discharging. The greater is the electrical capacity, the more heat is generated, and if the battery is sealed, the radiation of heat to outside the battery is slowed and even more heat builds up inside the battery, so when a battery pack with a large electrical capacity is made up of sealed rechargeable batteries, it is essential to provide some means for the efficient radiation of the heat that is generated. With the conventional rechargeable battery structure shown in FIG. 16, the coolant passages are formed by the ribs 8 between adjacent cells 1 as mentioned above, so heat generated by the cells 1 can be effectively radiated by forcing a coolant such as air through these coolant passages. A heat radiation structure such as this is disclosed in Japanese Laid-Open Patent Application No. 3-291867.
Nevertheless, when a battery pack is produced by arranging cells as in this conventional structure, a problem is that the greater is the number of cells 1 arranged, the greater is the temperature differential between the cells 1 located toward the middle and the cells 1 located on the outer ends. With the conventional structure shown in FIG. 16, the cells 1a and 1j located on the outer ends are less subjected to the effects of the heat generated by the other cells 1, and the end plates 32 also carry away some of the heat, so these cells are under good thermal radiation conditions. The closer a cell 1 is to the middle, the more it is affected by the heat generated by the cells 1 to either side, so the temperature rises more and heat radiation is not as good. Consequently, with a conventional structure, the closer a cell 1 is to the middle, the worse its heat radiation conditions are, the result being a temperature differential in which the temperature of the cells 1a to 1j is lower toward outside and higher toward the middle.
The charging efficiency of a rechargeable battery is affected by the temperature thereof, so if there is a temperature differential between the cells that make up the battery pack, as with a conventional structure, there will be a difference in the electrical capacity of the various cells. With a battery pack in which cells whose capacity thus varies are connected in series, those cells with lower capacity are in a state of overdischarge at the end of discharge. Repeated charging and discharging in this state in which there is a difference in the capacity of the cells shortens the cycling life of a battery pack and leads to diminished dischargeable capacity.
It is an object of the present invention to provide a rechargeable battery with enhanced performance as a battery pack, with no temperature differential between the cells that make up the battery pack.